Propellant compositions



United States Patent 3,000,714 PROPELLANT COMPOSITIONS George W.Batchelder, Glendora, and Gilbert A. Zimmerman, Monrovia, Calif.,assignors to Aerojet-General Corporation, Azusa, Calif., a corporationof Ohio No Drawing. Filed Dec. 21, 1953, Ser. No. 399,596 3 Claims. (Cl.'52-.5)

This invention relates to solid smokeless propellants useful for rocketpropulsion and has for its object to ilmprove and increase the burningrates of such propelants.

In the operation of jet motors it is customary to burn solid propellantcharges in the motor chambers to produce gas under pressure whichescapes at high velocity, generally through an exhaust nozzle leadingfrom the chamber, thereby producing thrust.

Generally propellants useful for this purpose comprise intimate mixturesof finely divided oxidizers, uniformly distributed in a resinous binderwhich acts as a fuel. The oxidizer is mixed with the monomer or lowermolecular weight, polymerizable unit of the fuel, and then polymerizedto produce a solid propellant grain.

In propellant grains commonly in use at present the oxidizer isordinarily a metal containing inorganic oxidizing salt such as potassiumperchlorate, ammonium dichromate, potassium nitrate, etc. The use ofthese salts produces burning rates of a suflicient magnitude to producethe required amount of thrust in combustion. However, in addition tobeing considerably more expensive than the non-metallic salts, thesemetal containing salts all possess one very undesirable property,namely, the production of large quantities of visible smoke due to theformation of metal oxides during combustion.

The use of non-metallic salts such a ammonium nitrates and ammoniumperchlorate in propellant grains is well known to eliminate theproduction of smoke, however, these oxidizing salts exhibit extremelylow burning rates and hence are unsuitable for use in propellantcompositions.

We have now found a class of burning rate catalysts whose presence innon-metallic grains will increase the burning rates of such compositionsby as much as 100%, thereby making practical the use of non-metallicinorganic oxidizing salts in solid propellants.

Substances useful for the acceleration catalyst of this invention aremixtures consisting of ammonium dichromate and silicon or titaniumcompounds capable of yielding oxides of those elements upon heating. Theoxides thus formed are believed to act as cracking catalysts for thecombustibles and provide increased surface area, thereby effectivelyincreasing the overall burning rate of the propellant as much as 100% insome cases.

Although some of the compounds employed in the burning rate accelerationcatalyst contain metal radicals, the elements are present in suchlimited quantities that they do not produce visible smoke duringcombustion.

The silicon and titanium compounds which are particularly useful in ourpropellants for improving burning rates are the lower alkyl siliconessuch as methyl, ethyl, propyl, isopropyl, etc., silicones and thecorresponding titanates such as methyl titanate, ethyl titanate, etc.

The particular organic radicals are of little importance since theactive groupings useful for the catalytic purpose are the and radicals.

Patented Sept. 19, 1961 Other siliceous compounds suitable for thispurpose are lower alkyl ortho silicates such as methyl, ethyl, propyl,isopropyl, etc., silicates; as well as inorganic silicates such ascalcium silicate, diatomaceous earth, silica gel, amine treated clay,and bentonite clay treated with aliphatic amines. In addition, loweralkyl and lower alkoxy siloxanes are also found to be useful.

Titanium compounds found to be useful for this purpose are the loweralkyl titanates.

Mixtures of these compounds and compositions have also been found to beuseful.

The burning rate acceleration catalyst is present preferably in anamount of from about 0.5% to 5.0% by Weight of the total propellantcompositions. Optimum results are obtained when the catalyst iscomprised of a mixture of from about 1.0 to 50.0% by Weight of ammoniumdichromate, the remainder being the siliceous or titanium component.

Suitable oxidizers for smokeless propellants are inorganic non-metallicsalts, as for example: ammonium and hydrazine salts of nitrates,chlorates and perchlorates. For the practice of this invention, thepreferred oxidizer is ammonium perchlorate.

The oxidizer should ordinarily be supplied in sufficient quantities atleast to oxidize all of the carbon in the resin fuel to carbon monoxideand /3 of the hydrogen to water. The amount of oxidizer added to theresinous mixture usually lies between 45% and by weight of the totalpropellant composition and the weight of the fuel mixture should liebetween 55% and 10% of the same propellant composition.

The fuel in which the oxidizer is dispersed is preferably a resincomprised of a polyester component, that is, the condensation product ofa polybasic carboxylic acid and a polyhydric alcohol, in which there isincorporated an olefinic component.

The polyester component, sometimes known as the alkyd component,ordinarily possesses some degree of unsaturation in the molecule inorder to permit polymerization with the olefinic component which may befor example; styrene, vinyl acetate, acrylic acid esters, methacrylicacid esters, allyl compound such as allyl diglycol carbonate, diallylmaleate, diallyl glycollate, and other unsaturated components such aspropylene butadiene, acetylene, etc; as well as derivatives of any ofthe above substances which are capable of polymerization with the resin.In general any olefin compatible with the resin and which willpolymerize with it is suitable. This includes all unsubstituted olefinsand in addition many substituted olefins. The unsaturation present inthe polyester permits the resulting unsaturated polyester to polymerizewith the double bond in the vinyl, allyl, or other olefinic additives.When a suiiicient amount of cross linkage occurs the resin becomesthermosetting. With a lesser degree of cross linkage the resin may bethermoplastic; and in some cases the resin possesses properties of boththermoplastic and thermosetting resins. All of these types of resins arewithin the purview of the present invention.

The polyester component can be made in general as follows: The hydroxygroups of dihydric or polyhydric alcohols are permitted to condense,with the polycarboxylic groups of, for example, a dicarboxylic acid, ora mixture of dicarboxylic acids, thereby producing an unsaturatedpolyester. Although the polyhydric alcohol and polycarboxylic acid willreact in stoichometric proportions, it is usually a better practice toemploy an excess of the alcohol.

The unsaturation permitting the polyester to polymerize with themonomeric vinyl, allyl or other olefinic component may be supplied byemploying either an unsaturated polyhydric alcohol or an unsaturateddicarboxylic acid. The usual and preferred manner is to condensemixtures of an unsaturated polycarboxylic acid or anhydride and asaturated or aromatic polycarboxylic acid,

or anhydride, with a polyhydric alcohol.

Saturated polycarboxylic acids useful in compounding the polyesterresins are, for example, the aliphatic dibasic acids, including: oxalic,malonic, succinic, glutaric, adipic, pimelic, sebacic, azelaic acids,etc., and the un saturated carboxylic acids useful as the acidiccomponents in forming polyester resins are maleic acid, fumaric acid,citraconic acid, mesaconic acid, itaconic acid, etc. The anhydrides suchas itaconic anhydride may likewise be used for supplying the desiredunsaturation.

Regardless of which of the saturated acids are used, the degree ofunsaturation necessary to provide cross linkage with the vinyl, allyl,or other olefinic components, may be obtained by the addition of any ofthe above-named unsaturated acids or their anhydrides.

Aromatic dibasic acids such as phthalic anhydride, naphthalic acid, andisomers of phthalic acid can be used in the same manner as theunsaturated aliphatic acids. These materials cannot be used as theunsaturated monomer inasmuch as aromatic compounds, althoughunsaturated, do not undergo addition polymerization.

The alcohols that can be used in this invention are not necessarilylimited to the dihydric alcohols, since other polyhydric alcohols, suchas the trihydric and higher polyhydric alcohols may also be used. Theseafiford additional possibilities for cross linking and as a consequencethe toughness and brittleness of the final resin may be controlled asdesired.

For the polyhydric alcohol component any of the following alcohols maybe used: dihydric alcohols, such as ethylene glycol, diethylene glycol,triethylene glycol, propylene glycol; a trihydric alcohol such asglycerol; tetrahydric alcohols such as the erythritols andpentaerythritols; pentitols which include arabitol, adonitol, xylitol;hexitols including mannitol, sorbitol, dulcitol; heptitols such aspersitol and volamitol, etc., or mixtures of any of the above alcoholsmay also be employed if desired.

The condensation of the polyhydric alcohol and polycarboxylic acid isusually conducted in the presence of the monomeric vinyl, allyl or otherolefinic component which form cross linkages between the polymericchains of the polyester. The amount of olefinic monomer may range fromabout 25% to about 100% by weight based on the weight of themonomer-resin mixture, and the amount of the monomer to be used in eachcase is determined by the particular properties which are desired in thefinished resin. In general, 50% by weight of polyester to 50% by weightof the olefinic additive produces a satisfactory polyester type ofmatrix for the propellant. The olefinic monomers listed above are allliquids and thereby serve as solvents for the heavier alkyd resin, thusfacilitating the dispersion of the oxidizer throughout the liquid resinbefore curing.

The propellant is preferably compounded as follows: The oxidizerrequired to oxidize the fuel to the proper degree is uniformly mixedinto the polyester resin-olefinic monomer mixture. Mixing is continuedat room temperature until all of the oxidizer has been added and themixture has a uniform consistency. A catalyst capable of acceleratingpolymerization of the resin mixture is added thereto and intimatelyincorporated therein before or at the time the oxidizer is added. In thesame fashion the burning rate expediting catalyst of the presentinvention is added to the monomer in finely divided form. All of thesesubstances including the oxidizer are thoroughly mixed with the liquidmonomer and the resultant mixture is then cast into a suitable mold andthe propellant substance cured.

The cast propellant is generally cured at temperatures ranging fromambient to about 220 F. When lower temperatures are employed the chargerequires considerable time to cure. If shorter cure times are desired a.

more elevated temperature can be used. The time and temperature forcuring makes no substantial difference in the ultimate product insofaras its ballistic properties are concerned.

A series of catalyzed propellants were investigated to determine theeffect of the herein described catalytic mixture and its variouscomponents upon the burning rate of representative propellant mixtures.The composition by weight percent of the propellant mixtures is asfollows:

Propellant Composition A: Weight percent Ammonium perchlorate 85.00

Polyester resin 3.58

Consisting of, by weight percent:

diethylene glycol 43.0 adipic acid 44.25 maleic anhydride 1.75 n-Butylacrylate 9.33 Methyl acryla 1.44 Methyl amyl ketone peroxide(polymerization catalyst) 0.40 Lecithin 0.25 100.00

Propellant Composition A: Weight percent Ammonium perchlorate 84.88Polyester resin 3.71

Consisting of, -by weight percent:

diethylene glycol--- 43.0 adipic acid 44.25

It may be observed from the foregoing table of burning rates that theburning rate of the propellant has been increased by the use of ammoniumdichromate-sihoeous catalysts more than Moreover, it is evident that thecatalytic effect of the ammonium dichromate-silicate mixture is greaterthan could be predicted from the catalytic behaviour of its respectivecomponents. The increase in burning rate induced by 1.5% of thecatalytic mixture provided by this invention is equal to the sum of theincreases effected by the individual use of its two substituents inamounts of 1% respectively.

From the foregoing discussion it is evident that the burning rateacceleration catalysts of the present invention will find valuable usein any gas generating propellant type compositions such asnitro-cellulose base propellants and other nitro substitutedresin-containing propellants.

It should be understood that the foregoing examples, although describingcertain specific embodiments of the invention, are provided primarilyfor purposes of illustration and are not intended to impose anylimitations upon its broader aspects.

We claim:

1. A solid propellant composition comprising a cured intimate mixture ofammonium perchlorate and a polyester resin consisting of thecondensation product of diethylene glycol, adipic acid, and maleicanhydride, heteropolymerized with n-butyl acrylate and methyl acrylate;said ammonium perchlorate being present in an amount of from about 45%to about 90% and said polyester resin being present in an amount of fromabout 55% to about and a mixture of from about 1.0% to about 50% byweight of ammonium dichromate and from about 99% to about 50% by weightof ethyl silicate in an amount of 2.0% by weight of the total propellantcomposition.

2. A solid propellant composition comprising a cured intimate mixture offrom about 45 to about 90% by weight of the total propellant compositionof a solid, nonmetallic, inorganic oxidizing salt, and from about 55% toabout 10% by weight of an unsaturated polyester resin consisting of thecondensation product of saturated polyhydric alcohol and polycarboxylicacid heteropolymerized with lower alkyl ester of lower alkenoic acid;and from about 0.5% to about 5.0%, for example, of a burning rateacceleration catalyst consisting of a mixture of from about 1.0% toabout 50% of ammonium dichromate and from about 99% to about 5 0% byweight of alkyl orthosilicate.

3. A solid propellant composition comprising a cured intimate mixture offrom about 45 to about 90% by weight of the total propellant compositionof a solid, nonmetallic, inorganic oxidizing salt, and from about 55% toabout 10% by weight of an unsaturated polyester resin consisting of thecondensation product of diethylene glycol, adipic acid and maleicanhydride heteropolymerized with a mixture of n-butyl acrylate andmethyl acrylate; and from about 0.5 to about 5.0% of a burning rateacceleration catalyst consisting of a mixture of from about 1% to aboutby weight of ammonium dichromate and from about 99% to about 50% byweight of alkyl orthosilicate.

References Cited in the file of this patent UNITED STATES PATENTS1,070,836 OBrien Aug. 19, 1913 2,159,234 Taylor May 23, 1939 2,388,319Fuller Nov. 6, 1945 2,472,963 Singleton et a1. June 14, 1949 2,479,828Geckler Aug. 23, 1949 2,555,333 Grand et al. June 5, 1951 FOREIGNPATENTS 248,089 Great Britain Mar. 1, 1926 579,057 Great Britain July22, 1946 OTHER REFERENCES Hackhs Chemical Dictionary, 3rd edition, The

Blakiston Co., Philadelphia, page 593.

1. A SOLID PROPELLANT COMPOSITION COMPRISING A CURED INTIMATE MIXTURE OFAMMONIUM PERCHLORATE AND A POLYESTER RESIN CONSISTING OF THECONDENSATION PRODUCT OF DIETHYLENE GLYCOL, ADIPIC ACID, AND MALEICANHYDRIDE, HETEROPOLYMERIZED WITH N-BUTYL ACRYLATE AND METHYL ACRYLATE,SAID AMMONIUM PERCHLORATE BEING PRESENT IN AN AMOUNT OF FROM ABOUT 45%TO ABOUT 90% AND SAID POLYESTER RESIN BEING PRESENT IN AN AMOUNT OF FROMABOUT 55% TO ABUT 10% AND A MIXTURE OF FROM ABOUT 1.0% TO ABOUT 50% BYWEIGHT OF AMMONIUM DICHROMATE AND FROM ABOUT 99% TO ABOUT 50% BY WEIGHTOF ETHYL SILICATE IN AN AMOUNT OF 2.0% BY WEIGHT OF THE TOTAL PROPELLANTCOMPOSITION.